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The excavation of a rural field in suburban Detroit has failed to turn up the remains of former Teamsters union leader Jimmy Hoffa, the FBI announced Wednesday, adding another unsuccessful chapter to a nearly 40-year-old mystery.

Authorities stopped the dig after just a few hours on the third day.

"We did not uncover any evidence relevant to the investigation on James Hoffa," said Robert Foley, head of the FBI in Detroit.

"I am very confident of our result here after two-days-plus of diligent effort," he said. "As of this point, we'll be closing down the excavation operation."

Authorities have pursued multiple leads as to Hoffa's whereabouts since his disappearance in 1975. He was last seen outside an Oakland County restaurant where he was to meet with a New Jersey Teamsters boss and a Detroit Mafia captain.

The latest tip about Hoffa's remains came from reputed Mafia captain Tony Zerilli, who, through his lawyer, said Hoffa was buried beneath a concrete slab in a barn in Oakland Township, north of Detroit.

The barn is gone, but FBI agents on Monday starting poring over the field where it used to stand.

On Tuesday, authorities used a backhoe to dig and move dirt around in the section of land. Authorities also called in forensic anthropologists from Michigan State University and cadaver dogs from the Michigan State Police.

He said about 40 agents were involved in an operation that covered about an acre. The FBI has not put a cost on the search, but Foley said it's more important to solve a case.

"With any investigation we consider cost-benefits analysis," he said. "The FBI and its partners are not corporations. We do not have a profit margin as a bottom line."

Hoffa's rise in the Teamsters, his 1964 conviction for jury tampering and his presumed murder are Detroit's link to a time when organized crime, public corruption and mob hits held the nation's attention. Over the years, authorities have received various tips, leading the FBI to possible burial sites near and far.

In 2003, a backyard swimming pool was dug up 90 miles northwest of Detroit. Seven years ago, a tip from an ailing federal inmate led to a two-week search and excavation at a horse farm in the same region. Last year, soil samples were taken from under the concrete floor of a backyard shed north of the city. And detectives even pulled up floorboards at a Detroit house in 2004.

No evidence of Hoffa was found.

Other theories have suggested he was entombed in concrete at Giants Stadium in New Jersey, ground up and thrown in a Florida swamp or obliterated in a mob-owned fat-rendering plant.

Zerilli, now 85, was in prison for organized crime when Hoffa disappeared. But he told New York TV station WNBC in January that he was informed about Hoffa's whereabouts after his release. His attorney, David Chasnick, said Zerilli is "intimately involved" with people who know where the body is buried.